The present invention relates generally to oils used as lubricants and, more specifically, to a method for evaluating such lubricants during use to determine their remaining useful life. The method is particularly adapted for use with ester-based oils.
It is common to lubricate and cool the components of operating equipment by wetting them with an oil lubricant. Such a lubricant experiences various environmental stresses as it carries out such functions that cause its basestock to undergo thermal-oxidative degradation. For this reason, various antioxidants are added to the lubricant to protect its lubricating characteristics. So long as the antioxidant system remains intact, the oxidative degradation of the basestock, and hence the changes in the lubricant's properties, are minimal.
Antioxidant species within a lubricating oil are gradually depleted with equipment operating time. Eventually, the antioxidants become ineffective, which allows large changes to occur in the physical properties of the lubricant's basestock. At such point, the lubricant is no longer capable of protecting the equipment and its useful life ends, resulting in excessive component wear and eventual failure of the equipment.
Since it is undesirable to continue to use a lubricant beyond the end of its useful life, scheduled lubricant changes have been devised for various types of operating equipment. The length of equipment operating time between scheduled changes is selected quite conservatively to ensure that dysfunctional lubricant is not permitted to remain within the equipment. However, this results in lubricants with remaining useful life being discarded.
It can therefore be seen that the ability to predict the remaining useful life of a lubricant would eliminate the need to perform lubricant changes on the basis of a fixed schedule. This would permit longer use of a lubricant, thereby providing savings in material and labor costs. Further, abnormal depletion rates for antioxidants within a lubricant sample can indicate severe wear problems prior to equipment failure.
Various thermal-oxidative and chemical-oxidative stressing techniques having the capability to evaluate remaining useful life are known. However, such techniques are unsuitable for routine use. Thermal-oxidative stressing techniques require the use of high temperatures and pressures and long analysis times in the order of 30 minutes. Chemical-oxidative stressing techniques are difficult in operation, require unstable reagents, and also require long analysis times in the order of 120 minutes.
One type of lubricant in common use is ester-based oils. Such oils are often used in gas turbine engines such as are typically found in aircraft. Ground turbine engines are utilized in power generation equipment. It would be advantageous to reduce the need for lubricant changes in such engines by evaluating the lubricant to determine its particular useful lifetime.
What is needed, therefore, is a method for evaluating the remaining useful life of a lubricant which does not require the use of high temperature and pressure or unstable reagents. The method should be specifically adapted for use with an ester-based oil lubricant. Such a technique should be rapid, i.e., analysis times of less than a minute, as well as easy to operate and capable of being performed with inexpensive equipment.